Methods and systems for the use of recycled filtrate in bleaching pulp

ABSTRACT

A system for the bleaching of lignocellulosic pulp including: a first oxygen reactor and a second oxygen reactor connected in series to the first oxygen reactor, wherein each reactor bleaches the pulp; an upstream pulp washer that washes pulp and discharges washed pulp to a first pulp conduit having an outlet coupled to the first oxygen reactor; an oxygen supply conduit connected to the first pulp conduit and supplying oxygen to the washed pulp flowing through the first pulp conduit; a downstream pulp washer that washes bleached pulp discharged from the second oxygen reactor; a filtrate conduit receiving filtrate discharged from the downstream pulp washer; a second pulp conduit receiving bleaches pulp discharged from the first oxygen reactor and conveying the bleached pulp to the second oxygen reactor, wherein the filtrate conduit is connected to the second pulp conduit such that at least a portion of the filtrate in the filtrate conduit flows into the second pulp conduit, and a second oxygen supply conduit connected to the second pulp conduit and introducing oxygen to the pulp flowing through the conduit.

RELATED APPLICATION

This application claims the benefit of U.S. Patent Application Ser. No. 60/891,552 filed Feb. 26, 2007, which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates the bleaching of cellulosic pulp. Systems and methods for bleaching cellulosic pulp are well known in the art, such as the one described in U.S. Pat. No. 7,156,952 to Ragnar.

FIG. 1 illustrates a prior art system 100 for the bleaching of cellulosic pulp using two oxygen reactors. A slurry of cellulosic pulp at a medium consistency of 8-18% enters the system via conduit 102, which feeds a first drum displacement (DD) washer 104. Wash liquid for the first washer is taken from filtrate recycled from downstream washer, such as second DD washer 138, flowing through via conduit 108. The first DD washer 104 discharges filtrate via conduit 106 and washed pulp via conduit 110. The washed pulp is fed to a mixer 112, where the pulp is mixed with oxygen from a conduit 114.

The mixture of washed pulp and oxygen enters a reactor 118 via conduit 116. In the reactor 118, the pulp has a residence time sufficient to at least partially bleach the washed pulp via oxygen bleaching. Via conduit 120, the pulp discharged from reactor 118 is fed into a second mixer 122, where the pulp is mixed with medium pressure steam received from conduit 124. The discharged pulp from mixer 122 is fed via conduit 126 to a mixer 128, where it is mixed with oxygen from a conduit 130. The mixture of steam, pulp, and oxygen enters a second reactor 134 via conduit 132. Similar to the first reactor 118, the pulp in second reactor 134 has a residence time sufficient to at least partially bleach the washed pulp via oxygen bleaching.

The discharge from the second reactor 134 is fed into the second DD washer 138 via conduit 136. Wash liquid (which may be filtrate) enters the DD washer 138 via conduit 140. Filtrate exits the DD washer 138 via conduit 108 and is recycled to the first DD washer 104. Washed and bleached cellulosic pulp exits the second DD washer 104 via conduit 142.

BRIEF DESCRIPTION OF THE INVENTION

A system has been developed to bleach pulp, preferably cellulosic pulp. The system may include at least two oxygen reactors for the oxygen bleaching of pulp. The system may include at least one washer (preferably a DD washer) that washes the bleached pulp after reaction in the second oxygen reactor. The system may further include a recycle conduit that recycles at least a portion of the filtrate exiting the washer. The recycle conduit may connect to a series of nozzles or to a mixer situated between two oxygen reactors. The series or nozzles or a mixer may mix the recycled filtrate with at least partially bleached pulp discharged from the first oxygen reactor and prior to introduction of the pulp in the second oxygen reactor.

A method has been developed for the bleaching of pulp, preferably cellulosic pulp. The method may include: feeding pulp (preferably washed pulp) to a first oxygen reactor for bleaching the pulp to generate an at least partially bleached pulp; mixing the at least partially bleached pulp with a recycled filtrate to generate a pulp-filtrate mixture; feeding the pulp-filtrate mixture to a second oxygen reactor for bleaching the pulp-filtrate mixture to generate a further bleached pulp; introducing the further bleached pulp to a washer for washing the further bleached pulp; and removing filtrate from the washer to generate the recycled filtrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art system for the bleaching of cellulosic pulp using two oxygen reactors.

FIG. 2 illustrates a system for the bleaching of cellulosic pulp using two oxygen reactors in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A system is disclosed to bleach pulp (which refers to generally any cellulosic material, such as wood fiber pulp) using two oxygen reactors. The system may be less complex and more reliable than prior art systems for oxygen bleaching of pulp. The inventive system may also improve the overall energy balance of the oxygen bleaching process.

FIG. 2 illustrates a system 200 for the bleaching of cellulosic pulp using two oxygen reactors 218, 234. A slurry of cellulosic pulp, preferably at a medium consistency of 8 percent to 18 percent (8-18%), enters the system via conduit 202, which is connected to a first drum dryer (DD) washer 204. Although illustrated as DD washer 204 in accordance with this exemplary embodiment, other washers (such as diffusion washers and pressurized washers) may be employed. Moreover, it is contemplated that a series or two of more washers may substituted for a single DD washer 204.

Filtrate recycled from another DD washer 238 enters the first DD washer 204 via conduit 203. The first DD washer 204 discharges filtrate via conduit 206 and washed pulp via conduit 210. The washed pulp is fed to a mixer 212, where it is mixed with oxygen received through gas conduit 214. The mixture of washed pulp and oxygen enters reactor 218 via conduit 216. The washed pulp has a residence time in the reactor 218 sufficient to at least partially bleach the washed pulp via oxygen bleaching. In certain embodiments, the pulp has a residence time of 15 minutes in the reactor 218.

Via conduit 220, the pulp discharged from the reactor 218 is fed to a series of nozzles or a mixer 221, where the pulp is mixed with filtrate recycled from the other DD washer 238 entering the series of nozzles or mixer 221 via conduit 223. The filtrate recycled from DD washer 238 may be heated in a heat exchanger 215, which transfers heat to the filtrate from steam entering via conduit 217 and exiting via conduit 219. Further, the filtrate may be pressurized via a pump 245. The pulp discharged from the nozzles or mixer 221 is fed to mixer 228, where it is mixed with oxygen received from conduit 230.

The mixture of filtrate, pulp, and oxygen enters a second reactor 234 from conduit 231, which is down stream from mixer 221. Similar to reactor 218, the pulp in reactor 234 has a residence time sufficient to at least partially bleach the washed pulp via oxygen bleaching. In certain embodiments, the pulp in reactor 234 has a residence time of 65 minutes or longer than the pulp residence time in reactor 218. The temperature (and other operational parameters) of the reaction occurring in the second reactor 234 may be controlled, at least in part, by the mixer 221, the flowrate of filtrate in conduit 223, and the temperature (and pressure) of the filtrate in conduit 223. Other factors influencing the control of the second reactor 234 may include: the flowrate content and consistency of the partially bleached pulp in conduit 220 from the first reactor 218, and the flowrate of oxygen in conduit 230.

The pulp discharged from the second reactor 234 is fed to the second DD washer 238 via conduit 236. Wash liquid, which may be filtrate, enters the second DD washer 238 via conduit 240. Filtrate exits the second DD washer 238 via conduit 208 and is divided at a splitter 211 into a first stream to be recycled to the first DD washer 204 via conduit 203 and a second stream directed to the mixer 221 via conduit 213. The recycled filtrate (line 208) may be split into streams in addition to the ones illustrated in FIG. 2. In alternative embodiments, the recycled filtrate 203 is not returned to DD washer 204, and the filtrate in conduit 203 may be discarded or consumed elsewhere in the pulping process. Washed and bleached cellulosic pulp 248 exits the second DD washer via conduit 242. The bleached pulp 248 may be further treated in additional bleaching processes and systems, similar to the system 200 illustrated in FIG. 2.

Although the above embodiment is described in connection with two reactors 218, 234, other embodiments including a single reactor or including three or more reactors are within the envisioned scope of the present invention. In an embodiment containing a single reactor, the filtrate is recycled to prior to introduction into the reactor.

Furthermore, process elements and design details, such as pumps, tanks, storage units, stand pipes, valves, heat exchangers, and other process elements not materially affecting the described systems and methods are not illustrated. These (and other) process elements merely tailor the particular configuration of the present invention to certain operation parameters. These details are a matter of design choice and necessity and are within the knowledge and abilities of ordinarily skilled artisans.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A system for bleaching of lignocellulosic pulp comprising: a first oxygen reactor and a second oxygen reactor each bleaching the pulp; a downstream pulp washer that receives and washes bleached pulp discharged from the second oxygen reactor; a filtrate conduit receiving filtrate discharged from the downstream pulp washer, and a pulp conduit receiving the pulp discharged from the first oxygen reactor and conveying the discharged pulp to the second oxygen reactor, wherein the filtrate conduit is connected to the pulp conduit such that at least a portion of the filtrate in the conduit flows into the pulp conduit.
 2. The system of claim 1 wherein the pulp washer is a drum displacement (DD) washer.
 3. The system of claim 1 further comprising an oxygen supply conduit connected to the pulp conduit and introducing oxygen to the pulp flowing through the conduit.
 4. The system of claim 1 further comprising a mixer coupled to the pulp conduit such that pulp flowing through the pulp conduit flows through the mixer and wherein the filtrate conduit is coupled to the mixer such that the filtrate flows into the mixer and enters the pulp conduit.
 5. The system of claim 1 further comprising an upstream pulp washer discharging washed pulp to a second pulp conduit connected to the first oxygen reactor.
 6. The system of claim 5 further comprising a second oxygen supply conduit connected to the second pulp conduit to introduce oxygen to the washed pulp in the second pulp conduit.
 7. The system of claim 5 wherein the filtrate conduit includes a branch of the conduit extending to a fluid inlet to the upstream pulp washer, and a portion of the filtrate in the filtrate conduit flows through the branch and into the fluid inlet of the upstream pulp washer.
 8. The system of claim 1 further comprising a heat exchanger coupled to the filtrate conduit and transferring heat to the filtrate flowing through the filtrate conduit.
 9. The system of claim 1 further comprising a pump coupled to the filtrate conduit and pumping the filtrate in the filtrate conduit towards the pulp conduit.
 10. A system for the bleaching of lignocellulosic pulp comprising: a first oxygen reactor and a second oxygen reactor connected in series to the first oxygen reactor, wherein each reactor bleaches the pulp; an upstream pulp washer that washes pulp and discharges washed pulp to a first pulp conduit having an outlet coupled to the first oxygen reactor; an oxygen supply conduit connected to the first pulp conduit and supplying oxygen to the washed pulp flowing through the first pulp conduit; a downstream pulp washer that washes bleached pulp discharged from the second oxygen reactor; a filtrate conduit receiving filtrate discharged from the downstream pulp washer; a second pulp conduit receiving bleaches pulp discharged from the first oxygen reactor and conveying the bleached pulp to the second oxygen reactor, wherein the filtrate conduit is connected to the second pulp conduit such that at least a portion of the filtrate in the filtrate conduit flows into the second pulp conduit, and a second oxygen supply conduit connected to the second pulp conduit and introducing oxygen to the pulp flowing through the conduit.
 11. The system of claim 10 wherein the upstream pulp washer and the downstream pulp washer are each a drum displacement (DD) washer.
 12. The system of claim 10 further comprising a mixer coupled to the second pulp conduit such that pulp flowing through the second pulp conduit flows through the mixer and wherein an outlet to the filtrate conduit is coupled to the mixer such that the filtrate flows into the mixer and thereby enters the second pulp conduit.
 13. The system of claim 10 wherein the filtrate conduit includes a branch of the conduit extending to a fluid inlet to the upstream pulp washer and a portion of the filtrate in the filtrate conduit flows through the branch and into the fluid inlet of the upstream pulp washer.
 14. The system of claim 10 further comprising a heat exchanger coupled to the filtrate conduit and transferring heat to the filtrate flowing through the filtrate conduit.
 15. A method for bleaching of cellulosic pulp comprising: introducing a pulp to a first oxygen reactor; bleaching the pulp in the first oxygen reactor and discharging from the first oxygen reactor at least partially bleached pulp; mixing the at least partially bleached pulp with a recycled filtrate to generate a pulp-filtrate mixture; introducing the pulp-filtrate mixture to a second oxygen reactor for bleaching the pulp-filtrate mixture and discharging a further bleached pulp; washing the further bleached pulp in a washer, and removing filtrate from the washer to generate the recycled filtrate.
 16. The method of claim 15 including adding oxygen to the partially bleached pulp or pulp-filtrate mixture after the partially bleached pulp is discharged from the first oxygen reactor and before the pulp-filtrate mixture is introduced to the second oxygen reactor.
 17. The method of claim 15 including washing the pulp and adding oxygen to the pulp before introducing the pulp to the first oxygen reactor.
 18. The method of claim 15 including heating the recycled filtrate before the mixing of the recycled filtrate with the at least partially bleach pulp.
 19. The method of claim 15 including pressurizing the recycled filtrate before the mixing of the recycled filtrate with the at least partially bleach pulp.
 20. The method of claim 15 including introducing a portion of the removed filtrate to a second washer and discharging from the second washer as washed pulp the pulp to be introduced to the first oxygen reactor. 